In most biological processes used today for sewage treatment, it is primarily the ability of microorganisms to oxidise organic toxins with oxygen of which use is made. For the operation of sewage treatment plants, therefore, the provision of a carrier material which offers bacteria favourable conditions for life in such plants is of great interest. In this regard, it is of decisive importance, on the one hand, that the largest possible surface be provided on which the bacteria can form colonies whilst, on the other hand, it is advantageous if the carrier material has a specific density which corresponds to that of the liquid to be treated, so that the carrier material becomes suspended in the liquid and can be circulated by existing currents. A further criterion for an advantageous bacteria carrier is the presence of a sufficiently smooth surface which enables the sludge containing the saturated bacteria building up on the material to fall off the carrier from time to time, providing room for new bacterial growth and thus for regeneration of the treatment unit.
It is known to use larva and/or shingle as carriers for bacteria in a fixed bed filter. This variant, however, is distinguished by the disadvantage that the carrier is denser than water and is therefore not capable of becoming suspended. A further disadvantage is that a high level of pressure loss occurs. Also disadvantageous for the use of these materials as bacteria carriers in sewage treatment plants is the fact that the contamination problem is not satisfactorily solved in this case, and that carrier materials of this type have too small a surface area.
Furthermore, the use of plastics carriers, particularly based on moulded bodies, in sewage treatment plants is known. These are distinguished, however, by a small surface area of up to a maximum of 600 m2/m3, and their partial tendency to float.
Carriers based on ceramics are also used, although these are also unsatisfactory, since on the one hand they are not capable of becoming suspended and, on the other hand, they are too expensive to produce.